There are several focus detection methods for cameras. Japanese Patent Laid-Open Nos. 55-111928 and 58-24105 disclose apparatuses for performing pupil division focus detection by using a two-dimensional sensor in which microlenses are formed on the respective pixels of the sensor.
Canon Inc. has proposed in Japanese Patent Laid-Open No. 2001-124984 an apparatus for performing pupil division focus detection by using an image sensor (image sensing element) used in a digital still camera.
FIG. 12 is a view for explaining the principle of a method of performing pupil division focus detection by using an image sensor, which is disclosed in Japanese Patent Laid-Open No. 2001-124984. An image sensor 10 is arranged on the prospective imaging plane of an image sensing lens 5. One pixel of the image sensor 10 is made up of two light-receiving portions 13a and 13b. The light-receiving portions 13a and 13b are so arranged as to be almost conjugate to the pupil of the image sensing lens 5 through a microlens 11 formed on the image sensing lens side.
The light-receiving portion 13a receives a beam having passed through a predetermined lower region of the image sensing lens 5 in FIG. 12. The light-receiving portion 13b receives a beam having passed through a predetermined upper region of the image sensing lens 5 in FIG. 12. In detecting the focus, signals from the light-receiving portions 13a and 13b of pixels are independently read out. These signals form two images of beams having passed through different positions on the pupil of the image sensing lens 5. Japanese Patent Laid-Open No. 5-127074 discloses a method of detecting a focus by using two images generated by beams having passed through different regions on the pupil of an image sensing lens. The image sensor disclosed in Japanese Patent Laid-Open No. 2001-124984 outputs the sum of an output from the light-receiving portion 13a of one pixel and an output from its light-receiving portion 13b in general photography.
Japanese Patent Publication No. 5-61610 discloses a method of controlling exposure of a focus detection sensor in detecting a focus. To monitor the light quantity incident on the focus detection sensor, the focus detection sensor of this reference has monitoring pixels which are interposed between pixels for generating a focus detection image and monitor the light quantity incident on the sensor. Outputs from the monitoring pixels control the accumulation time of the focus detection pixel and the like.
The image sensor for performing pupil division focus detection is constituted such that a light-receiving portion in one pixel of the image sensor is divided into two regions. The light-receiving sensitivity is low in the region between these light-receiving portions.
FIGS. 13A and 13B are views showing a beam incident on one pixel of the image sensor in general photography. In FIGS. 13A and 13B, reference numerals 13a and 13b denote two divided light-receiving portions. The light-receiving sensitivity is low in the region between these light-receiving portions. The dotted circuit in FIG. 13A is an incident beam when the stop of the image sensing lens is in a full aperture state. When the stop is in a full aperture state, a beam incident on the low-sensitivity region is less than beams incident on the light-receiving portions 13a and 13b. The degree of decrease in image sensor output is small, hardly influencing exposure control.
If the stop of the image sensing lens is stopped down, the ratio of a beam incident on the low-sensitivity region to beams incident on the light-receiving portions 13a and 13b increases, as shown in FIG. 13B. The degree of decrease in image sensor output increases. When photometry is done with the image sensing lens being in a full aperture state and the stop of the image sensing lens is stopped down in accordance with the photometry result, the image sensor output decreases more than the stopping-down effect.
The light-receiving portion of the image sensor and the pupil of the image sensing lens are almost conjugate to each other through the microlens. The pupil of the image sensing lens and the light-receiving portion of the sensor have the same relationship as that shown in FIG. 13A on the pupil plane of the image sensing lens. Beams incident on the light-receiving portions 13a and 13b of the image sensor have a semicircular pupil shape (portion where the pupil (circle represented by the dotted line) of the image sensing lens and the light-receiving portions (rectangles represented by solid lines) overlap each other). This shape is different between the light-receiving portions 13a and 13b (mirror image relationship).
FIG. 14 shows the image sensor outputs of one white line image generated by the light-receiving portions 13a and 13b of the image sensor. An image a in FIG. 14 is an image formed by outputs from the light-receiving portions 13a of pixels. An image b in FIG. 14 is an image formed by outputs from the light-receiving portions 13b of pixels. In this image sensor, the similarity between the two images a and b is low. Even correlation calculation using the two images produces a calculation result error, generating a focus detection error.
To detect the focus state of the image sensing lens, monitoring pixels for determining the charge accumulation time of the pixels and the like are desirably arranged near pixels for acquiring a focus detection image, and a high-contrast focus detection image is desirably acquired. However, the above image sensor does not incorporate any monitoring pixels for determining the charge accumulation time of the pixels and the like in order to detect the focus state of the image sensing lens. In the image sensor, the accumulation time and the like are controlled based on an output from a separate photometry sensor. A high-contrast image is not always obtained.